1. Field of the Invention
The present invention relates to a load control device for a light-emitting diode (LED) light source, and more particularly, to an LED driver for controlling the intensity of an LED light source.
2. Description of the Related Art
Light-emitting diode (LED) light sources are often used in place of or as replacements for conventional incandescent, fluorescent, or halogen lamps, and the like. LED light sources may comprise a plurality of light-emitting diodes mounted on a single structure and provided in a suitable housing. LED light sources are typically more efficient and provide longer operational lives as compared to incandescent, fluorescent, and halogen lamps. In order to illuminate properly, an LED driver control device (i.e., an LED driver) must be coupled between an alternating-current (AC) source and the LED light source for regulating the power supplied to the LED light source. The LED driver may regulate either the voltage provided to the LED light source to a particular value, the current supplied to the LED light source to a specific peak current value, or may regulate both the current and voltage.
The prior art dealing with LED drivers is extensive. See, for example, the listing of U.S. and foreign patent documents and other publications in U.S. Pat. No. 7,352,138, issued Apr. 1, 2008, assigned to Philips Solid-State Lighting Solutions, Inc., of Burlington, Mass., and U.S. Pat. No. 6,016,038, issued Jan. 18, 2000, assigned to Color Kinetics, Inc., of Boston, Mass. (hereinafter “CK”).
LED drivers are well known. For example, U.S. Pat. No. 6,586,890, issued Jul. 1, 2003, assigned to Koninklijke Philips Electronics N.V., of Eindhoven, the Netherlands (hereinafter “Philips”), discloses a driver circuit for LEDs that provide power to the LEDs by using pulse-width modulation (PWM). Other examples of LED drivers are U.S. Pat. No. 6,580,309, published Sep. 27, 2001, assigned to Philips, which describes switching an LED power supply unit on and off using a pulse duration modulator to control the mean light output of the LEDs. Moreover, the aforementioned U.S. Pat. No. 6,016,038 also describes using PWM signals to alter the brightness and color of LEDs. Further, U.S. Pat. No. 4,845,481, issued Jul. 4, 1989 to Karel Havel, discloses varying the duty cycles of supply currents to differently colored LEDs to vary the light intensities of the LEDs so as to achieve continuously variable color mixing.
U.S. Pat. No. 6,586,890 also discloses a closed-loop current power supply for LEDs. Closed-loop current power supplies for supplying power to other types of lamps are also well known. For example, U.S. Pat. No. 5,041,763, issued Aug. 20, 1991, assigned to Lutron Electronics Co., Inc. of Coopersburg, Pa. (hereinafter “Lutron”), describes closed-loop current power supplies for fluorescent lamps that can supply power to any type of lamp.
U.S. Pat. No. 6,577,512, issued Jun. 10, 2003, assigned to Philips, discloses a power supply for LEDs that uses closed-loop current feedback to control the current supplied to the LEDs and includes means for protecting the LEDs. Likewise, U.S. Pat. No. 6,150,771, issued Nov. 21, 2000, assigned to Precision Solar Controls Inc., of Garland, Tex., and Japanese patent publication 2001093662A, published Apr. 6, 2001, assigned to Nippon Seiki Co., Ltd., describe over-current and over-voltage protection for drivers for LEDs and other lamps.
LED drivers that may be dimmed by conventional A.C. dimmers are also known. Thus, aforementioned U.S. Pat. No. 7,352,138, and U.S. Pat. No. 7,038,399, issued May 2, 2006, assigned to CK, describe LED-based light sources that are controlled by conventional A.C. phase control dimmers. The aforementioned U.S. Pat. No. 6,016,038 discloses a PWM controlled LED-based light source used as a light bulb that may be placed in an Edison-mount (screw-type) light bulb housing. Control of lamps, such as LED lamps, by phase control signals are also described in U.S. Pat. No. 6,111,368, issued Aug. 29, 2000, U.S. Pat. No. 5,399,940, issued Mar. 21, 1995, U.S. Pat. No. 5,017,837, issued May 21, 1991, all of which are assigned to Lutron. U.S. Pat. No. 6,111,368, for example, discloses an electronic dimming fluorescent lamp ballast that is controlled by a conventional A.C. phase control dimmer. U.S. Pat. No. 5,399,940 discloses a microprocessor-controlled “smart” dimmer that controls the light intensities of an array of LEDs in response to a phase control dimming voltage waveform. U.S. Pat. No. 5,017,837 discloses an analog A.C. phase control dimmer having an indicator LED, the intensity of which is controlled in response to a phase control dimming voltage waveform. The well-known CREDENZA® in-line lamp cord dimmer, manufactured by Lutron since 1977, also includes an indicator LED, the light intensity of which is controlled in response to a phase control dimming voltage waveform.
Applications for LED illumination systems are also shown in U.S. Pat. No. 7,309,965, issued Dec. 18, 2007, and U.S. Pat. No. 7,242,152, issued Jul. 10, 2007, both assigned to CK. U.S. Pat. No. 7,309,965 discloses smart lighting devices having processors, and networks comprising such smart lighting devices, sensors, and signal emitters. U.S. Pat. No. 7,242,152 discloses systems and methods for controlling a plurality of networked lighting devices in response to lighting control signals. Such systems are also used in the RADIORA® product, which has been sold since 1996 by Lutron.
In addition, there are known techniques for controlling current delivered to an LED light source. LED light sources are often referred to as “LED light engines.” These LED light engines typically comprise a plurality of individual LED semiconductor structures, such as, for example, Gallium-Indium-Nitride (GaInN) LEDs. The individual LEDs may each produce light photons by electron-hole combination in the blue visible spectrum, which is converted to white light by a yellow phospher filter.
It is known that the light output of an LED is proportional to the current flowing through it. It is also known that LEDs suffer from a phenomena known as “droop” in which the efficiency is reduced as the power is increased. For LEDs of the GaInN type (used for providing illumination), a typical load current is approximately 350 milliamps (mA) at a forward operating voltage of between three and four volts (V) which corresponds to approximately a one watt (W) power rating. At this power rating, these LEDs provide approximately 100 lumens per watt. This is significantly more efficient than other conventional light sources. For example, incandescent lamps typically provide 10 to 20 lumens per watt and fluorescent lamps, 60 to 90 lumens per watt. As discussed, LED light sources can provide larger ratios of lumens per watt at lower currents, thus avoiding the droop phenomena. Further, it is expected that, as technology improves, the efficiency of LED light sources will improve even at higher current levels than presently employed to provide higher light outputs per diode in an LED light engine.
LED light sources typically comprise a plurality of individual LEDs that may be arranged in both a series and parallel relationship. In other words, a plurality of LEDs may be arranged in a series string and a number of series strings may be arranged in parallel to achieve the desired light output. For example, five LEDs in a first series string each with a forward bias of approximately 3 volts (V) and each consuming approximately one watt of power (at 350 mA through the string) consume about 5 W. A second string of a series of five LEDs connected in parallel across the first string will result in a power consumption of 10 W with each string drawing 350 mA. Thus, an LED driver would need to supply 700 mA to the two strings of LEDs, and since each string has five LEDs, the output voltage provided by the LED driver would be about 15 volts. Additional strings of LEDs can be placed in parallel for additional light output, however, the LED driver must be operable to provide the necessary current. Alternatively, more LEDs can be placed in series on each sting, and as a result, the LED driver must also be operable to provide the necessary voltage (e.g., 18 volts for a series of six LEDs).
LED light sources are typically rated to be driven via one of two different control techniques: a current load control technique or a voltage load control technique. An LED light source that is rated for the current load control technique is also characterized by a rated current (e.g., 350 milliamps) to which the peak magnitude of the current through the LED light source should be regulated to ensure that the LED light source is illuminated to the appropriate intensity and color. In contrast, an LED light source that is rated for the voltage load control technique is characterized by a rated voltage (e.g., 15 volts) to which the voltage across the LED light source should be regulated to ensure proper operation of the LED light source. Typically, each string of LEDs in an LED light source rated for the voltage load control technique includes a current balance regulation element to ensure that each of the parallel legs has the same impedance so that the same current is drawn in each parallel string.
In addition, it is known that the light output of an LED light source can be dimmed. Different methods of dimming LEDs include a pulse-width modulation (PWM) technique and a constant current reduction (CCR) technique. Pulse-width modulation dimming can be used for LED light sources that are controlled in either a current or voltage load control mode. In pulse-width modulation dimming, a pulsed signal with a varying duty cycle is supplied to the LED light source. If an LED light source is being controlled using the current load control technique, the peak current supplied to the LED light source is kept constant during an on time of the duty cycle of the pulsed signal. However, as the duty cycle of the pulsed signal varies, the average current supplied to the LED light source also varies, thereby varying the intensity of the light output of the LED light source. If the LED light source is being controlled using the voltage load control technique, the voltage supplied to the LED light source is kept constant during the on time of the duty cycle of the pulsed signal in order to achieve the desired target voltage level, and the duty cycle of the load voltage is varied in order to adjust the intensity of the light output. Constant current reduction dimming is typically only used when an LED light source is being controlled using the current load control technique. In constant current reduction dimming, current is continuously provided to the LED light source, however, the DC magnitude of the current provided to the LED light source is varied to thus adjust the intensity of the light output.
There is a need for an LED driver that that is able to provide smooth, flicker-free dimming of the LED light source using constant current reduction dimming, particularly, in the event of changes in the desired intensity of the LED light source.